1. Field of the Invention
This invention relates to a system for measuring temperature and relative humidity and, more specifically, to an electronic, low cost relative humidity temperature measurement system.
2. Brief Description of the Prior Art
Some of the most reliable, cost effective and miniature relative humidity sensors available are based upon water absorptive polymers. The major drawbacks to these sensors include temperature sensitivity, the need for low cost AC stimulation and non-linear response to relative humidity. These problems dictate that electronics must be employed to condition the sensor's response. To compensate for temperature sensitivity, these electronics must employ some form of temperature measuring device, such as a thermistor. The use of AC stimulation to the relative humidity sensor brings present two distinct problems. First, almost all highly integrated electronics are powered by DC voltages. This implies that the DC power must be converted to AC power for the sensor. The amplitude of the AC power is also temperature sensitive unless some form of amplitude feedback control is used, the latter generally being expensive. Second, stimulation of a relative humidity sensor with AC power requires that the AC signal from the sensor be converted back to DC. This requires filtering and, usually, rectification. The non-linear response of the the relative humidity sensor requires signal linearization and calibration. Correction of these problems is generally, quite expensive.
For HVAC application or any "comfort control", it is always desirable to measure temperature in addition to relative humidity. In general, this requires an additional temperature sensor and output circuitry.